You could (R-Click on layer in timeline) add a mask to the layer, shape it and feather it. Than add Effects/Render/Fill to adjust the mask color to match the background, if necessary.
Create another mask on the layer (or on a duplicate layer Control+D ) to mask the head.
Keyframe the shape of the masks to match any motion, if necessary.
You could use View/Motion Tracker (or is it Window/Motion Tracker??)if appropriate.
Import your 3d text footage. Adjust the Premultiplied or Alpha Chanel settings as necessary, during import. If you had the option to save your 3d footage with some sort of mask or transparency, that is good:)
If not, you’ll need to key out the background of the 3d text, maybe a Effects/Keying/Color Key and Effects/Matte/Matte Choker?